F.A.Q. (en)


PM2.5 & Pollutrack

1Why are airborne particles dangerous?

The smaller, the more dangerous. Fine particles PM2.5 of less than 2.5 microns - twenty times smaller than hair - can penetrate very deep in the lungs and for the smallest ones even invade the blood stream through the alveoli…

Fortunately, not all particles are dangerous. The larger ones, such as pollen, sand and most dusts are unpleasant and may sometimes cause allergies, but their health effects are limited.

However, a very specific category of particles, called PM2.5 also known as Fine Particles, which measure less than 2.5 microns (20 times smaller in diameter than a fine hair) are particularly harmful because their small size allows them to penetrate deeply into the respiratory tract. They contribute significantly to asthma and other major respiratory morbidities including Chronic Obstructive Pulmonary Disease, and potentially lung cancer even for non-smokers who have been over exposed to these particles.

The smallest of these particles measuring below 1 micron have even the capacity to cross the membrane separating the capillaries from the pulmonary alveoli. Once in the blood stream, where they remain for a long time, these particles can cause blood disease, strokes, major cardiac disorders and digestive cancers. They are also known to be responsible for foetal damage in pregnant women.

2Why have we started measuring fine particles PM2.5 on a large scale only recently?

Because of a major and most disruptive innovation: laser microsensors are now available at a much lower cost than the existing high precision instruments, which cost was limiting their deployment. In a matter of three years, Paris has moved from a single PM2.5 urban Fixed Station (Place Stravinsky, Beaubourg) into a dense network of hundreds of mobile laser microsensors continuously autochecking their respective PM2.5 measurements.

The particles have always been followed-up regularly, but with a limited number of monitoring stations, due to the high cost of the precision instruments required for the fine weighing of particles. In Paris, there are only three such reference stations for PM2.5. One is located at Place Stravinsky in Beaubourg, and two more referred to as “Traffic Stations” are located on the Paris ring road, in the west at Porte d'Auteuil and in the east nearby Saint-Mandé. These three stations comply in full with European air quality monitoring directives, but now have the advantage of being complemented by a finer network of laser-based monitoring systems throughout the city centre.

This additional network has become possible due to the emergence of a new technology that is much more cost effective: Laser Particle Counting.

Laser technology makes it possible to measure in real time the number of particles in each neighbourhood, street by street, and the measurements are acquired at head height thereby collecting more “real life data” than purely static weighing systems. Accordingly, Pollutrack in partnership with Airparif and the electricity distributor Enedis, who have made available their entire fleet of 300 electric cars in Paris, are measuring these particles in liaison with the “Fondation du Souffle”, a group of expert doctors specializing in respiratory and cardiac pathologies, and very familiar with how air quality affects health.

The additional contribution of 100 Renault Zoe cars from the Marcel Cab Company allows the possibility of tracking 24 hours a day including weekends, thereby creating a total fleet of 400 electric vehicles equipped with laser sensors within the Paris areas.

3In short, what is Pollutrack all about?

400 electric vehicles continuously monitoring air quality at breathing level across the streets of Paris, H24 and 7 days a week. A unique system with no equivalent worldwide.

300 electric vehicles from the Enedis fleet travel the streets of Paris daily as part of their regular activities but the innovation results from the fact that laser sensors for fine particles have been installed throughout this professional fleet and these can now be observed on the tops of their blue vehicles. Each laser counts the number of particles surrounding each vehicle every second and sends an average data set every ten seconds by GSM, to ensure the reliability of the measurement.


Pollutrack in Paris achieves an historic first: The additional contribution of around 100 Renault Zoé from the VTC Marcel (Renault) fleet makes it possible to obtain 2 million PM2.5 counts each day and a geographical recording of nearly 200,000 qualified data sets daily.

The accumulation of these measurements, taken at breathing height, makes it possible to know precisely, taking into account the topology specific to each neighbourhood, the exposure to particles of pedestrians including cyclists and joggers and especially children and babies in strollers, who are among the most fragile and yet most exposed individuals -, and finally anyone who has to travel in the city's hypercentre.

4What lessons can we learn from this?

Plenty and even more each day of tracking. It’s a whole new world, this breakthrough making the invisible visible. It will help defining the necessary Low Emission Zones, and finally the Ultra-Low Emission Zones (ULEZ) wherever hotspots of PM2.5 are identified.

There are many, but we must now take the time to exploit the tens of millions of data sets that have already been accumulated. These surveys will make it possible to identify neighbourhoods, streets, or even sections of streets, where exposure is above average either because of traffic or because of the street configuration, including the so-called Canyon Effect which is where a relatively narrow street may be dominated by disproportionately tall buildings which favours the accumulation of fine particles due to lack of air circulation. Other sources of pollution other than traffic can also be identified, such as certain subway exits or even subway air vents, construction sites, diesel bus warehouses and any other source of fine particles.
5Ultimately what outcomes can we expect?

By looking very early at the progressive accumulation of PM2.5, this new tool brings to the City the unprecedented possibility of reacting before the formation of a pollution peak instead of waiting days until larger and older particles PM10 eventually trigger the alarm…

The ongoing exploitation of these first two years of data sets made through the streets of Paris shows that the phenomenon of PM2.5 particles is more complex than it seems.

The main sources of primary PM2.5 emissions are already well known and the city is actively working to reduce them in several ways. These include encouraging residents to use their vehicles less, encouraging the use of shared vehicles for journeys, encouraging all ecomobility alternatives, offering incentives to taxi drivers/companies for the purchase of clean vehicles, by regulating traffic as soon as a peak starts to form, instead of mostly intervening too late as was regularly the case until this year...

But a more complex and insufficiently documented phenomenon to date requires particular attention: this is the formation of secondary fine particles of PM2.5, which result from the remote chemical recombination of exhaust gases, a recombination that occurs far behind vehicles. These resulting fine particles are therefore not taken into account by the current air quality model algorithms, due to their complexity and unpredictable behaviour. This is precisely where real life and real time measurements street by street at breathing level are superior to static weighing systems…

6How and why are secondary PM2.5 particles challenging?

Their formation is the result of complex gas recombinations far away from cars, very much depending on many factors such as temperature, humidity, wind speed and direction etc. Due to this complexity, existing models focus less on secondary pollution, hugely variable by nature, and that can only be monitored through real life and real time measurements.

This the key advantage of the Pollutrack approach. As much as the formation and origin of primary PM2.5 particles are already known and therefore manageable (mandatory particulate filters for all new vehicles, eradication of older vehicles, incentives to change boilers and not to use an open fireplace etc..), the knowledge of secondary PM2.5 is in its infancy, because it is infinitely more complex to establish the secondary PM2.5 data and requires better technology on a mobile and real time basis to be able to take the required measurements.

The level of accumulation, movement and transformation of secondary PM2.5 particles arising from exhaust gases (especially the large quantities of diesel NO2, known since “Dieselgate”, with emissions data having been rigged by many car manufacturers) depends on wind, sunlight, gas concentration, temperature, humidity and many other factors that vary from season to season, day to day and even hour to hour...

Only these millions of "Real Life" and real-time data sets of PM2.5 levels at breathing height carried out with Pollutrack will enable us to monitor, quantify and then control the places where these secondary particles are formed or accumulated, sometimes unexpectedly.

7Can we already locate PM2.5 hotspots?

Yes, and very clearly so. The coming months will be dedicated to qualifying the sources of primary pollution and even more importantly, locating the various spots where secondary pollution accumulates under specific circumstances to be precisely defined.

Next step will consist in eradicating these spots wherever possible.

It is still too early to draw definitive conclusions, but some major trends are clearly emerging after two continuous years of fine particle tracking.

Although the primary sources of PM2.5 particle production are inevitably located in busy city areas, such as the inner-ring road, major intramural traffic routes, construction sites, large warehouses, parking areas for diesel buses and taxis, tourist coaches and finally the last remaining diesel TERs in SNCF stations, the distribution of secondary pollution, as we have seen above, is much more complex.

In essence, secondary PM2.5 hotspots "follow the wind", and secondary pollution only settles in Paris in particularly calm and sunny weather, especially in winter when the temperature curve reverses (the bell effect). Therefore the overall PM2.5 level, accounting for both primary + secondary pollution, is higher in winter, not only because of the temperature and the fact that vehicles are more polluting in cold weather, but also because anticyclonic conditions predominate.

As an illustration, it is remarkable that as soon as air circulates at less than 5 km/h above the capital in good weather, the risk of a pollution peak materializes: it takes less than an hour for secondary particles to form, i.e. at this air speed it can move about 5km which is approximately commensurate with the radius of the Paris metropolitan area...

In other words, if a weak wind blows from south-west to north-east, which is the predominant direction of wind over Paris, exhaust gases from the south ring road will be found in many areas across the Right Bank of the Seine River, in the form of secondary fine particles PM2.5.

However, as soon as the wind speed exceeds 10 km/h, the dispersion of pollution is significantly increased resulting in the secondary pollution falling outside of the Paris metropolitan area radius and not within the radius. Accordingly, this wind effect has an impact on neighbouring cities, so pollution in Paris does not only concern Parisians...

Once the predominant location and dynamics of the secondary PM2.5 urban pollution hotspots are precisely known, it will then be possible to consider the pedestrianization of certain streets, particularly those with a “canyon effect”, or potentially making entire districts "zero emission areas", for instance nearby overexposed schools, as London has just begun to do. This is to avoid the harmful combination of Primary Pollution from local traffic & activities and Secondary Pollution from peripheral traffic.

These districts will then be reserved for ecomobility with a global ban on all combustion engines, from scooters to delivery vehicles, enjoying a significantly improved air quality....

8Simply put, do we breathe well in Paris?

Not so bad at all… After all, Paris has the highest population density of the western world and the sixth highest worldwide. But regarding the most important PM2.5, we are still too often above the WHO daily limit. Reducing the number of days above it is our top priority.

Actually not too bad, when you take into account that Paris, with its 2,200,000 residents over 105 km² and its 50 million visitors each year is the most densely populated city in the Western world...

Paris is also the sixth highest population density in the world, so the problem of air quality is not easy to manage. Nevertheless, our annual average PM2.5 remains in a good shape when compared to international averages.

However, the number of days exceeding the PM2.5 ceiling of maximum 25 micrograms of particles per m3 of air over 24 hours set by the World Health Organization must be drastically reduced. We still cross this threshold about forty times a year, whereas the WHO recommends that we limit ourselves to three days annually... Paris' objective is to become a "WHO Breathable City" by moving closer to the organization's recommendations, which is far more stringent than the European Commission regulations which are still under the diesel lobby influences, and we will do our utmost to achieve this.

So there is still work to be done, and we are working on it with great determination after deploying Pollutrack through the streets of the capital, and making Paris the best equipped city in the world in terms of fine particle detection and monitoring.

9Do Parisians understand your approach to improving air quality?

Gradually, but there is still a lot of education to be done, especially with "urban sportsmen" who practice their favourite sports discipline sometimes at peak pollution times, when they inhale 6 to 12 times more air than at rest... It also remains to look more closely at the exposure of children to air pollution in schools, which varies greatly from one district to another...

10Finally: what can be done individually?

Individually, fight unceasingly and with determination against car-solo, the dreadful single-person car driving. Convince yourself but also your family and friends...

The average occupancy rate of a vehicle in Paris is 1.1 people, one of the lowest in the world. This grotesque situation is totally unacceptable in a city with such a comprehensive public transport networks and the most modern alternatives for soft mobility.

Let's be optimistic, less than 10% of Parisians use their vehicle during the week, and less than 40% of them own a car, the use of which is mainly dedicated to holidays and weekends. The Parisian is also the city dweller who travels the most on foot, representing 50% of intramuros trips, a fact that is also unique in the world...

This unfortunately means that Paris congestion, and therefore intramuros pollution, originates from outside the city. Alongside the many Ile de France Mobilités work sites that are dragging on in Paris intramuros, the lack of car parks at the gates of the Capital or near train and subway stations is a real problem that the Region has never had the will to resolve, and which must become a priority for Greater Paris if the pollution situation is to improve significantly.

We are looking at new and effective ways to discourage this single occupancy car use and encourage car sharing or carpooling. Free-floating and new, highly accessible applications will gradually contribute to this.

Cycling is also developing as a result of the multiplication of secure bicycle paths on increasingly long journeys. We will also encourage all delivery actors to drive cleanly, or even to adopt electric tricycles for the hypercentre, most exposed to the steady development of e-commerce.

Finally, let us be clear: the car itself is not a problem in the city, if it is clean, compact and shared, and if it avoids the neighbourhoods where walking is a real legitimate pleasure.

The days of big SUV’s occupied by a single individual - 80% of car-solo are men – are numbered…